The invention concerns a pressure measuring device.
In pressure measurement, a distinction is made between differential, absolute and relative pressure measuring devices. Differential pressure measuring devices serve for measuring the difference between two different pressures. In the case of absolute pressure measuring devices, a pressure to be measured is sensed absolutely, i.e. as a pressure difference with respect to a vacuum. With a relative pressure measuring device, a pressure to be measured is recorded in the form of a pressure difference with respect to a reference pressure. The reference pressure is an ambient pressure at the measuring location. In most applications, this is the atmospheric pressure at the place where the device is being used.
Pressure measuring devices usually have a housing, a pressure measuring cell enclosed in the housing for sensing the pressure and an electronic circuit. The pressure measuring cells have at least one pressure-sensitive diaphragm, on the outer side of which a pressure acts during operation. There are on the market, for example, non-metallic pressure measuring cells, in which the pressure acts directly on a measuring diaphragm. The measuring diaphragms generally consist of insulating materials, such as glass, ceramic or sapphire for example, so that an electromechanical transducer can be arranged directly on the measuring diaphragm. The electromechanical transducer converts the mechanical deflection of the measuring diaphragm into an electrical value, which is then available to the electronics for further evaluation and/or processing.
Measuring devices are powered by means of a commercially available transmitter power-supply unit, in an industrial environment usually without a ground connection, and an output signal is picked up for example by means of a load arranged at a location remote from the measuring device. Power supply lines and/or signal lines may be of a considerable length, so that there is the risk of electromagnetic interference being introduced. The introduction of so-called common mode interference, in which electromagnetic interference acts simultaneously on the connecting cables of the measuring device with respect to a common reference system, for example ground, is particularly critical. As a consequence of such common mode interference, common mode interference currents flow via the connecting cables of the measuring device, its electronic circuit and its housing to the reference system.
In the case of pressure measuring devices there is additionally the risk of interference currents flowing via the pressure measuring cell and these interference currents being superposed on the unamplified and/or unconditioned measured variables and/or measuring signals. This may lead to considerable measuring errors.
In the article xe2x80x98Der Einfluxcex2 von Gleichtaktstxc3x6rungen auf industriell eingesetzte Sensoren und Mexcex2systemexe2x80x99 [The influence of common mode interference on industrially used sensors and measuring systems] by H. Waldschmidt, which was published in the report xe2x80x98Sensoren und Mexcex2systemexe2x80x99 [Sensors and measuring systems] issued by the VDI Verlag GmbH in 1996 to accompany the conference held in Bad Nauheim from Mar. 11 to 13, 1996, there is a description of a pressure measuring device which comprises:
a housing
a pressure measuring cell
which has at least one pressure-sensitive measuring diaphragm,
on the outer side of which a pressure acts during operation and
which has a transducer for converting a pressure-dependent deflection of the measuring diaphragm into an electrical measured variable, and
an electronic circuit for converting the electrical measured variable into a measuring signal.
The pressure measuring cell is, as described at the bottom of page 523 and represented in FIG. 4, enclosed in a housing and is laterally surrounded in the housing by a shielding made of a copper foil insulated on both sides. The shielding forms as it were a housing in the housing and is connected to a reference potential, for example to an input of the pressure measuring device. An interference signal, introduced from outside the housing, consequently flows essentially via the shielding to the reference potential and the pressure measuring cell remains largely unaffected by interference.
Just such a pressure measuring device is also described in EP-A 780 674.
However, capacitive connections, the capacitances of which are usually referred to as stray capacitances, exist between the pressure measuring cell and the shielding and between the pressure measuring cell and the housing. The magnitude of the stray capacitances depends on the spatial arrangement of the pressure measuring cell, the shielding and the housing in relation to one another and on the dielectric constant of a medium located between the components mentioned. It is usually air, the dielectric constant of which depends on its moisture content at any given time. Therefore, although externally introduced interference is dissipated essentially via the shielding, a small interference current, dependent on the magnitude of the stray capacitances, also flows via the pressure measuring cell, where it may be superposed with the interference-sensitive measured variable and/or the interference-sensitive measuring signal. The measuring accuracy therefore depends among other things on the spatial arrangement of the pressure measuring cell, shielding and housing in relation to one another and on the dielectric constant of the medium at any given time. In the case of capacitive pressure measuring cells in particular, i.e. pressure measuring cells in which a capacitance changing as a function of pressure is measured, these variable stray capacitances may be superposed on the capacitance to be measured and consequently lead to a change in the measuring signal of the pressure measuring device.
The shielding represents an additional component which has to be produced and fitted. Moreover, a thin copper foil is mechanically sensitive and dimensional changes have an effect on the magnitude of the stray capacitances.
It is an object of the invention to specify a pressure measuring device with a housing and a pressure measuring cell arranged in it in which the pressure measuring cell is protected from interference, in particular independently of its installation position in the housing.
For this purpose, the invention constitutes a pressure measuring device which comprises:
a housing
a pressure measuring cell
which has at least one pressure-sensitive measuring diaphragm,
on the outer side of which a pressure acts during operation,
which has a transducer for converting a pressure-dependent deflection of the measuring diaphragm into an electrical measured variable, and
which has free outer circumferential surfaces,
which are provided with an electrically conductive coating, and
an electronic circuit, arranged in the housing, for converting the electrical measured variable into a measuring signal.
According to one embodiment, the electrically conductive coating is a sputtered-on metallic coating.
According to one embodiment, the electrically conductive coating is a metallic lacquer, in particular a conductive carbon lacquer or a conductive silver lacquer.
According to one embodiment, the electrically conductive coating is a layer in a laminated foil.
According to one development of the invention, the pressure measuring cell is a capacitive pressure measuring cell with an electrode arranged on the measuring diaphragm and, together with the electrode, the coating forms a closed Faraday cage.
According to one embodiment, the electrode is connected via the coating to a reference potential.
According to one development of the invention, the pressure measuring cell is a capacitive differential pressure measuring cell which has two measuring diaphragm with electrodes arranged on them, and, together with the electrodes, the coating forms a closed Faraday cage.
According to one embodiment, the electrodes are connected via the coating to a reference potential.
The invention and its advantages are now explained in more detail with reference to the figures of the drawing, in which two exemplary embodiments are represented. The same elements are provided with the same reference numerals in the figures.